1. Field of the Invention
The present invention relates to a solid-state image sensor having a function of decreasing a shading amount, a production method for the solid-state image sensor, and a digital camera using the solid-state image sensor, and particularly to a solid-state image sensor wherein micro-lenses are placed on the photodetecting cells belonging to the incident side, the production method of the solid-state image sensor, and a digital camera using the solid-state image sensor.
2. Description of the Related Art
Recently, video cameras and digital cameras have become wide-spread in general. CCD-type or MOS-type solid-state image sensors are used in these cameras. In such solid-state image sensors, a plurality of photodetecting cells having a light-receiving part (a photoelectric converter) are arranged to form a matrix. The energy of the light incident to each photodetecting cell undergoes photoelectric conversion in the light-receiving part, generating a signal charge. The generated signal charge is outputted to the external parts, via a CCD and a signal channel.
As shown in FIG. 32, a CCD-type solid-state image sensor 10 of the related art has a photodetecting cell 13 having a light-receiving part 2, a vertical CCD 15 and a horizontal CCD 16 constituting the light-receiving part 2 for transferring signal charges, and an output amplifier 17.
Among the photodetecting cells placed in the light receiving area (the area to which the light is incident) of the solid-state image sensor 10, there are valid cells wherein the energy of the light, incident to the light-receiving part 2, undergoes a photoelectric conversion into a signal charge for outputting said signal charge, and there are photodetecting cells for outputting dark currents without photoelectric conversion.
A photodetecting cell which outputs dark currents is known, for instance as a black dummy. Such a photodetecting cell has an incident side which is shielded from the light, and is generally placed in one row and/or one column surrounding the valid cell area wherein a plurality of valid cells are arranged to form a matrix, or in a row at the extremity of any side of the valid cell area where a plurality of valid cells are arranged to form a matrix.
In addition, the solid-state image sensor 10 is equipped with a light-blocking layer such that the light is only incident to the light-receiving part 2 of the valid cells, and with signal driving channels for applying voltage to CCD electrodes, not shown in FIG. 32.
In addition, color filters are placed above each light-receiving part, for taking color pictures with the solid-state image sensor 10.
FIG. 33 is a layout view showing an example of an array of color filters. The plurality of color filters together forms a layer. R, G and B represent red, green and blue filters, respectively. One of the R, G or B filters is placed above the light-receiving part 2.
In addition, to improve the converging power, a micro-lens is sometimes placed above each light-receiving part 2. FIG. 34 is a cross-sectional view showing the structure of a photodetecting cell of the solid-state image sensor 10 of the related art.
In the above-mentioned solid-state image sensor 10, the light-receiving part 2 is formed on top of the semiconductor substrate 1 (for example a silicon substrate), a light blocking layer 9 with apertures 8 is placed on the incident side of the light-receiving part 2.
Above each light-receiving part 2, one of the R, G and B color filter 4 is placed in an on-chip fashion. In addition, a micro-lens 7 for improving the converging power is placed immediately above the light-receiving part 2, via a flattening layer 6.
In fact, in such a solid-state image sensor 10, a phenomenon called shading is known which gives rise to sensitivity fluctuations in the valid cell area.
Shading originates from the fact that incident lights incident to the peripheral part of the valid cell area, when compared to incident lights incident to the central part of the valid cell area, have incidences which are oblique. In other words, when the light is obliquely incident, it generates eclipses and a degradation of the photoelectric conversion rate at the level of the light-receiving part 2.
In this case, since the quantity of incident lights in the central part of the valid cell area is greater, for the same quantity of incident lights, the output signal is greater for the photodetecting cells of the central part when compared to the photodetecting cells of the peripheral part. Therefore, a xe2x80x9csensitivity fluctuationxe2x80x9d is generated between the photodetecting cells of the central part and the photodetecting cells of the peripheral part. Also, in this document, the xe2x80x9csensitivity fluctuation (or the difference in the output value)xe2x80x9d is called the shading amount. The shading amount increases when the number of photodetecting cells increases, and the size of the valid cell area increases.
FIG. 35 shows an example of results obtained when the shading amount is measured for a solid-state image sensor.
The measurements of the shading amount shown in FIG. 35 have been obtained by measuring the output of a valid cell area, whose size was 25.1 mm in the horizontal direction and 16.9 mm in the vertical direction.
In the drawing, xcex94 is the G output voltage of the central part (sensitivity, equivalent to the actual aperture rate), xe2x97xaf is the calculated value for the latter, xc3x97 is the actual measurement of the G output voltage for the peripheral part, and xe2x96xa1 is the calculated value of the latter.
From this figure, it is clear that the shading amount between the central part and the peripheral part depends on the F number of the digital camera, the result of which is displayed as the difference in the G output voltage (sensitivity).
A so-called xe2x80x9cmicro-lens positional offsettingxe2x80x9d method, wherein the center of micro-lens belonging to the peripheral part is moved towards the central part of the valid cell area, taking the center of the corresponding light-receiving part as the reference, and a method wherein the aperture width of the light blocking layer is larger the closer to the periphery it is, taking the center of the corresponding light-receiving part as the reference, have been proposed as methods for decreasing such shading amounts.
Of these, the xe2x80x9cmicro-lens positional offsettingxe2x80x9d is publicly known, for instance, as disclosed in Japanese patent No. 2600250.
In the xe2x80x9cmicro-lens positional offsettingxe2x80x9d, as shown in FIG. 36, the center of a micro-lens 27 installed above a light-receiving part (for example a photovoltaic such as a photodiode) 22, is matched with the center of a light-receiving part 22 (double-broken lines in the drawing) in case the light-receiving part belongs to the central part 21A of the valid cell area 21, and offset by a specified distance d1 towards the central part of the valid cell area 21, in case the light-receiving part belongs to the peripheral part 21E of the valid cell area.
The specified distance d1 is defined so it becomes greater at a constant rate, the further from the center 21X of the solid-state image sensor 20. In addition, optimal values are determined for the specified distance d1, taking into consideration the characteristics of the camera lenses and the solid-state image sensor 20 actually used. In addition, in the drawing, numeral 23 is an inter-level isolation layer, numeral 24 is a color filter and numeral 26 is a flattening layer.
The xe2x80x9cmicro-lens positional offsettingxe2x80x9d shading countermeasure mentioned above has been recognized to be effective to some extent, but it is still not sufficient. The reasons are explained concretely in the following.
First, the above-mentioned shading countermeasure has the problem of not taking into account the solid-state image sensors wherein color filters are placed on the incident side, thereby generating color shading due to said color filters. Color shading designates the offset of color balance between the central part and the peripheral part.
Second, when applying the xe2x80x9cmicro-lens positional offsettingxe2x80x9d, which offsets the position of the lenses, to actually-made solid-state image sensors, it has not been possible to decrease the shading to the same extent as the values calculated in simulations. In addition, the aperture area of the light blocking layer mounted in the solid-state image sensor has not been taken into consideration. In other words, although when the aperture area of the light blocking layer is wider, the light leak increases and leads to such a problem as cross-talk, which is one of the effects due to shading, and a malfunction of switch transistors, these phenomena have not been taken into account in the xe2x80x9cmicro-lens positional offsettingxe2x80x9d.
Third, when designing in shading countermeasures, the characteristics of the digital camera, wherein the corresponding solid-state image sensor is applied, have not been taken into account. In other words, the F number of the camera lens equipped in digital cameras, and/or the actual F number change the angle of incidence of the light with respect to the light-receiving part, and this F number dependency of the incident angle influences the shading amount.
In the related art, taking into account this F number dependency of the shading amount, a correction to increase the brightness of the peripheral part by an image processing device installed in the camera side, has been considered (a software shading correction). This shading correction is performed as the step 1 of image processing (FIG. 37) executed by the computer of a digital camera carrying the solid-state image sensor 10. However, to execute the shading correction program, it is normally necessary to equip the digital camera side with a special control circuitry, which raises the costs. In addition, when the shading amount is large, by executing this shading correction, the efficiency of other processes requiring faithful color reproduction degrades and causes the problem of the image itself becoming unnatural. In addition, when the shading value is large, depending on the performance of the computer loaded onto the camera, rapid image processing can be difficult. The defect becomes a larger problem for CCD-type solid-state image sensors with an increased valid cell area.
In addition, the above-mentioned F number dependency of shading becomes particularly problematic, in the case of exchangeable lens-type digital still cameras, when the camera lens unit is substituted (when exchanging lenses).
Furthermore, the decrease in the shading amount by xe2x80x9cmicro-lens positional offsettingxe2x80x9d is limited in the width of the offset, since it is a method wherein correction is made by offsetting the position of the micro-lens with respect to the position of the light-receiving part (photovoltaics). This is particularly a problem for large-size solid-state image sensors (film-size CCD-type solid-state image sensors) wherein the degree of oblique incidence is extremely large, and where not enough offset width can be maintained.
The present invention has been conceived in view of the above, and its first object is to provide a solid-state image sensor which can decrease even color shading, and/or a solid-state image sensor with superior shading effects, and a digital camera using such solid-state image sensor.
The second object of the present invention is to provide a solid-state image sensor which decreases shading independent from the aperture reserved at the light blocking layer, and/or a solid-state image sensor with superior shading effects, and digital camera using such solid-state image sensor.
In addition, the third object of the present invention is to provide a solid-state image sensor wherein the shading amount of a solid-state image sensor can be decreased while lowering the F number dependency, in a simple structure.
In addition, the fourth object of the present invention is to provide a solid-state image sensor wherein the shading amount of the solid-state image sensor can be decreased accurately in response to an actual environment of a digital camera taking pictures.
In order to achieve the above-mentioned objects, the solid-state image sensor of the present invention comprises a valid cell area wherein a plurality of light-receiving parts and a plurality of valid cells having color filters placed in an on-chip fashion corresponding to the light-receiving parts and outputting charge signals, are arranged to form a matrix, the color filters placed in the peripheral part of the valid cell area are offset with respect to the light-receiving parts in the direction to the center of the valid cell area, and the offset amounts between the color filters and aforementioned light-receiving parts become gradually or continuously larger, the further it is from the center and the closer it is to the periphery of the valid cell area. The above configuration makes it possible to appropriately offset the color filters with respect to the position of the light-receiving parts, and color mixture is decreased even when oblique incident light components are present.
Preferably, in the solid-state image sensor, the valid cell area is divided into groups of a plurality of concentric blocks, with the offset amount between the color filters and the light-receiving parts being the same within each block, and increasing from the central part to the peripheral part. The above configuration not only allows a decrease in color mixture, but also allows use of a relatively low-cost reticle as a reticle for making color filters when producing solid-state image sensors, and also decreases production costs.
In addition, another solid-state image sensor of the invention comprises a valid cell area wherein a plurality of light-receiving parts and a plurality of valid cells having color filters placed in an on-chip fashion to correspond to the light-receiving parts and outputting charge signals, are arranged to form a matrix, the color filters placed in the peripheral part of the valid cell area are offset with respect to the light-receiving parts in the direction to the center of the valid cell area, the light-receiving parts and the color filters are placed on a respective constant pitch, with the pitch for the light-receiving parts being greater than the pitch for the color filters. This configuration makes it possible to make the difference of the offset amounts between the color filters and the light-receiving parts continuous from the central part to the peripheral part of the valid cell area. Images obtained from such solid-state image sensors present more natural photographic subjects.
In addition, another solid-state image sensor of the present invention comprises a valid cell area, having a plurality of valid cells, each comprising a light receiving part and a light blocking layer in which apertures are provided corresponding to the light-receiving part, for outputting charge signals arranged to form a matrix, the apertures reserved at the peripheral part of the valid cell area are offset with respect to the light-receiving parts in the direction of the center of the valid cell area, and the offset amounts between the apertures and aforementioned light-receiving parts become gradually or continuously larger, the further it is from the center and the closer it is to the periphery of the valid cell area. The above configuration makes it possible to appropriately offset the apertures with respect to the position of the light-receiving parts, and the shading amount can be decreased without generating eclipses, even when oblique incident light components are present.
Preferably, the valid cells of the solid-state image sensor are divided into groups of a plurality of concentric blocks, with the offset amount between the apertures and the light-receiving parts being the same within each block, and the offset amounts being larger as it gets further from the central part and closer to the peripheral part. The above configuration not only allows a decrease in the shading amount, but it also allows use of a relatively low-cost reticle for making the apertures of a light blocking layer, and decreasing the production costs.
In addition, yet another solid-state image sensor of the present invention is equipped with a valid cell area wherein a plurality of light-receiving parts and a plurality of valid cells having a light blocking layer in which apertures are reserved to correspond to the light-receiving parts and outputting charge signals, are arranged to form a matrix, the apertures reserved at the peripheral part of the valid cell area are offset with respect to the light-receiving parts in the direction to the center, the light-receiving parts are placed and the apertures are reserved with a respective constant pitch, with the pitch for the light-receiving parts being greater than the pitch for the apertures. This configuration allows to make the difference of the offset amounts between the apertures of the light blocking layer and the light-receiving parts continuous from the central part to the peripheral part of the valid cell area, thus, the images obtained present more natural photographic subjects.
In addition, when considering the application of each of the above-mentioned solid-state image sensors of the present invention to digital cameras, the offset between the center of the light-receiving parts and the center of the micro-lenses equipping the incident side, the offset between the center of the light-receiving parts and the center of the color filter, and the offset between the center of the light-receiving parts and the center of the apertures can be determined based on the total thickness of layers, the thickness of the layer between the light-receiving parts and the layer equipped with the micro-lenses, the thickness of the layer between the light-receiving parts and the color filters, the thickness of the layer between the light-receiving parts and the apertures, the refractive index of the layer placed underneath the micro-lenses and the eye-relief of the optical system equipped in the digital camera.
In addition, yet another solid-state image sensor of the present invention has a valid cell area formed by a plurality of photodetecting cells made of a plurality of photovoltaics arrayed on the main side of a semiconductor substrate, in which, according to the position of the photovoltaics inside the valid cell area, on the light receiving side of the corresponding photovoltaic, a penetration adjusting device is placed to adjust the optical penetrating amount of the corresponding incident light. This allows proper reduction in the shading amount which differs according to the position of the valid cell area.
Preferably, the penetration adjusting device of the solid-state image sensor is a layer made of organic materials, formed in the upper part of the photovoltaics, which has different optical penetration amount according to the position inside the valid cell area. This allows, by only changing the optical penetrating rate of the layer made of organic material, composition for an optimal decrease of the shading amount without modification of other structures.
In addition, preferably, in the solid-state image sensor, micro-lenses are configured in the layer made of organic material, formed on the light receiving side of the photovoltaics, and have different optical penetrating rates depending on the position from the peripheral part to the central part of the valid cell area. This allows, by only changing the optical penetrating rate of the micro-lenses, in other words, without modification of other structures, composition for an optimal decrease of the shading amount.
In addition, preferably, in the solid-state image sensor, micro-lenses are placed on the light receiving side of the photovoltaics, with the layer made of organic material serving as the flattening layer formed between the photovoltaics and the micro-lenses. This allows, by only changing the optical penetrating rate of the flattening layer, in other words, without modification of other structures, composition for an optimal decrease of the shading amount.
When producing such solid-state image sensor, a layer is formed, wherein the optical penetrating rate changes according to the irradiation amount due to ultra-violet rays, and the layer is selectively exposed to ultra-violet rays in an amount which changes according to the position of the valid cell areas. This allows, by only adjusting the amount of ultra-violet irradiation occurring during the generally performed xe2x80x9cadded exposurexe2x80x9d, production of a solid-state image sensor with an optimal decrease of the shading amount.
In addition, preferably, a layer is formed wherein the optical penetrating rate changes according to the temperature of heating, and the layer is heat-treated, according to the position of the valid cell areas. This allows, by only adding a simple production process, production of a solid-state image sensor with an optimal decrease of the shading amount.
In addition, preferably, the penetration adjusting device is placed on the light receiving side of the valid cell area, and functions as an optical penetrating rate controlling device capable of controlling the optical penetrating rate. Fine control (decrease) of the shading amount is possible using the optical penetrating rate controlling device.
In addition, preferably, in the solid-state image sensor, the optical penetrating rate controlling device controls the optical penetration rate based on the signals from the brightness sensor mounted in the surrounding of the valid cell area. This allows for a proper and optimal decrease of the shading amount according to the actual environment of a digital camera taking pictures.
Solid-state image sensors configured as mentioned above are mounted into digital cameras. This allows for correction of the shading according to the environment of a digital camera taking pictures.